The electrical discharge machining is using electrodes to machine a workpiece with the principle of electrical discharge and high temperature melting, wherein the discharge loop of the traditional current-limited discharge power source comprises a 220V AC power source, an AC power transformer T1, a bridge rectifier, current-limited resistance R1, a diode D5 and a transistor Q1 (shown as FIG. 1). The AC power transformer is stepping down the 220V AC-grid to AC 56-70V, and then the AC 56-70V is rectified and filtered to DC 80-100V by the bridge rectifier to limit the current with the current-limited resistance R1. In the meanwhile, a high-voltage DC power is produced between electrodes when the transistor Q1 is conducted, and a discharge current destroyed insulation is produced when the gap is reaching suitable distance and the discharge current becomes zero till the transistor Q1 is cut-off. After the cut-off time of the transistor Q1, the transistor Q1 is conducted again, and then the conducting time and cut-off time of the transistor Q1 is cycled to acquire a series of discharge currents. The disadvantage of this kind of power source is that the volume of the AC power transformer T1 is larger with higher power of the AC power transformer T1. In the aspect of loss, because the loss power of the current-limited resistance R1 is the product of the resistance and the square of the current, the loss power is extremely high and the Watt of the current-limited resistance R1 is relatively higher to result in the problems of large volume and hard cooling. The actual power used for electrical discharge machining is 30%, and the rest of power (70%) is wasted in the current-limited resistance R1 to result in low efficiency.
Please refer to TW publication patent No. 201025821, which is disclosed a green-energy power generator for electrical discharge machine. When the electrical discharge machine starts to machine, a high-voltage DC power arc is produced first, and then the output voltage (discharge voltage) of an AD adaptor is step-down to achieve the objective of power saving. However, the discharge voltage is not adjustable, so that the cutting requirement of the week conductive and variable resistance materials is not able to be resolved.
Please refer to TW publication patent No. 201006595, which is disclosed a self-tuning power control method and device for discharge machining power system. Low-voltage ignition and high-voltage discharge are used, wherein the low-voltage ignition is tunable, but the pulse width of the power source of the discharge machining is narrow and easily resulting in inefficient discharge machining.
Please refer to U.S. Pat. No. 4,071,729, which is disclosed an adaptive control system and method for electrical discharge machining. It comprises a current limiting stage for limiting the peak current provided to the gap at certain critical combinations of on-off time ratios so that possibility of gap short circuiting is substantially eliminated.
Please refer to U.S. Pat. No. 4,460,815, which is disclosed a current limit system and method for electrical discharge machining apparatus. The system operates to use the desired on-time and current values, but the off-time will be modified depending on the maximum current limit possible. This will protect the apparatus when an impossible combination has been entered by the operator. The method insures that the frequency to current limit ratio is within workable limits. The method includes a comparing the operator preset values to a predetermined value. If the operator preset period (off-time plus on-time) is greater than that value, the machining operation proceeds. If the period is less than that value, a minimum allowable period is calculated by taking the current limit number and dividing by 4 and 8. The results are added to determine the base minimum period. The desired or preset period is then subtracted from the minimum period and the difference added to the off-time. Electrical discharge machining then proceeds.
Please refer to U.S. Pat. No. 5,371,334, which is disclosed a method of electrical discharge machining control by monitoring gap resistance. A pair of electrodes are spaced from one another so as to define a gap. One of the electrodes constitutes a workpiece to be machined and the other electrode constitutes a machining tool. To machine the workpiece, groups of electrical discharges are generated between the electrodes. The different groups are separated by pauses which are free of discharges, and the electrical resistance of the electrode gap is measured during the pauses. When the resistance decreases to a critical value, the lengths of the pauses and the spacing between the electrodes are increased until the resistance begins to increase from the critical value. Electrical discharge activity is continued during this time. In the event that these measures fail to cause an increase in the resistance so that the critical value is obtained during several pauses, the electrical discharges are stopped and the machining electrode is abruptly shifted away from the workpiece electrode. The distance moved by the machining electrode is proportional to the frequency of repetition of the critical value and to the period of time for which the resistance remains at such value. The objects of the method are provided to make it possible to increase the rate of erosion during electrical discharge machining and be capable of reducing wear of the machining electrode during electrical discharge machining.
Please refer to U.S. Pat. No. 4,710,603, which is disclosed an electrical discharge machining power source. It is capable of eliminating the conventional drawback and to provide an electrical discharge machining power source which can charge a capacitor discharge circuit at a high speed, which eliminates heat dissipation and which provides high power source efficiency. Furthermore, it is capable of preventing a charging switching element from a capacitor in the capacitor discharge circuit from being damaged by a surge voltage and decreasing a stray inductance present in a power source circuit and to minimize energy loss.